In many electronic circuit applications, it has been determined that the circuit components do not operate properly because of variations of humidity and temperature in the atmosphere which surrounds these circuit components. It is well understood that resistance values of resistors, capacitance values of capacitors, etc. in fact change in response to variations in temperature and humidity in the ambient air. It follows that these changes in circuit values of resistance and capacitance, etc. are highly undesirable since they lead to improper circuit performance.
Electronic components which are used in space vehicles, for instance, must be of a high precision nature and must not be infirmed by changes in values due to ambient temperature and humidity conditions. Accordingly, there are military specifications which set down the standards of humidity and temperature that such electronic components must meet in order that said components can be employed in circuitry used in space vehicles.
One attempt to isolate electronic components from the effects of temperature and humidity comprised encapsulating said electronic components in plastic material. For instance, it was hoped, early in the development of sealed components, that encapsulating components in an epoxy resin would serve to keep said components free from the effects of temperature and humidity changes. Unfortunately, it became apparent that the temperature coefficient of the epoxy usually differed from that of the component that it was surrounding and, accordingly, there resulted undesirable stresses on both the epoxy as well as the components as a result of changes in temperature. Secondly, it was often the case that moisture would creep through an epoxy encapsulation which was subjected to repeated cycles or conditions of high humidity.
In an effort to overcome the foregoing problems at least two major techniques have been attempted. First, it has been determined that moisture will not readily pass through a ceramic material. Glass and steatite are typical examples of ceramic materials which are non-porous. Accordingly, as part of one of the techniques found in the prior art, a hollow cylindrically-shaped ceramic substrate is employed and a thin film of metal is secured or deposited on the inside surface (i.e., the surface of the hollowed out section of the cylinder) to form an electrical resistance path. In other words, the thin film of metal conducts current but in so doing, acts as an electrical resistor. Such a resistor element, of course, is not a standard resistor inasmuch as a standard thin film electrical resistor normally has the film secured to the outside of the substrate. In addition, in the first technique, the hollowed out section is filled with an inert gas and end caps are secured over the cylinder ends. The end caps are formed so as to be in electrical connection with the metal film path which is disposed along the inside surface of said hollowed out portion of said cylinder. The sealed resistor of this first technique works reasonably well (when it is properly fabricated) even when subjected to variations of temperature and humidity. However, this last described resistor assembly is very costly and somewhat difficult to fabricate, since the deposition of the metal thin film on the inside of said hollowed out section and the spiralling thereof is a difficult and costly technique. In addition, this first technique (and its resultant resistor assembly) has severe limitations in that it does not lend itself to the production of a large number of different resistor sizes, and is limited in the field of miniaturized resistors since there is a lower practical limit on the size of a hollow cylinder which can have a spiral pattern of metal deposited therein.
In the second major technique, mentioned earlier, a standard resistor is located within a glass housing. Since the coefficient of temperature of the standard resistor normally would differ from the coefficient of temperature of the glass, a "bellows wire" (e.g., a loop wound, or a repeatedly crimped, wire which can expand when subjected to heat) is used to electrically connect the end of the standard resistor to the outside lead-in wire. This bellows wire acts as a support means to support one end of the standard resistor within the glass housing and also acts as an electrical connection to the "outside world." When there are changes of temperature, the bellows wire either expands or contracts depending upon the temperature condition and this enables the package to withstand temperature changes with a minimum amount of stress. However, the package is not a very ruggedized package and cannot withstand, for instance, great vibrations. In addition, it is also a costly matter to fabricate this last-mentioned package because of the "bellows wire" and because of the particular manner in which the lead-in wires must be secured to the glass housing.